When trying to insult someone or just make a joke, it's usually better to use "stronger" sentences that have more of an impact on the audience, and when one compares metaphors (a connection between two unlike things [i.e., You ARE babies] to say that one has traits of the other; in this case, a baby's cowardice) to similes (a connection between two like things [i.e., You are LIKE babies] to emphasize one thing's traits; in this case, a person's cowardice is being likened to that of a baby), metaphors are stronger, making a joke/insult more effective.
TL;DR No. Why? Because the creators of the English language want to mess with everyone.
It should always be Babys in German.
This lesson that introduces the word Baby doesn't seem to have any exercises where the plural is spelled anything but Babys, but it's possible that the word is used in later lessons as well and that the incorrect plural Babies might have been added to the list of accepted alternatives there.
If you do find that other sentence, please report the mistake and/or comment on that sentence discussion.
In english you say "don't be a baby", "you're being a big baby". I guess for a group you could say "you are all a bunch of babies".
It shows 'Ihr seid Babys' as 'You are babies' though. I get that the 'Ihr seid' implies plural, so is this like saying "you're a bunch of babies" or "You guys are acting like babies"? Because as an English speaker, Duo's translation of "You are babies" sound very weird, unnatural, and not grammatically correct. I have never heard anyone say that... anyone else?